1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, and more particularly to an evaporative fuel-processing system which has a function of detecting abnormalities in control valves provided in an evaporative emission control system of the engine.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines, which comprises a canister having an air inlet port provided therein, a first control valve arranged across an evaporative fuel-guiding passage extending from a fuel tank of the engine to the canister, and a second control valve arranged across a purging passage extending from the canister to an intake system of the engine.
A system of this kind temporarily stores evaporative fuel in the canister, and then purges the evaporative fuel into the intake system of the engine.
Whether a system of this kind is normally operating can be checked, for example, by bringing the evaporative emission control system into a predetermined negatively pressurized state, measuring a change in the pressure within the fuel tank (tank internal pressure) with the lapse of time after the evaporating emission control system has been brought into the predetermined negatively pressurized state, and determining whether the system is normally operating, from the measured tank internal pressure, as proposed by U.S. Ser. No. 07/942,875 assigned to the assignee of the present application.
According to the method of the earlier application, a third control valve is provided at the air inlet port of the canister, for closing and opening the same. To detect an abnormality in the evaporative emission control system, the third control valve is kept closed and the first and second control valves are kept open while the engine is operating so that the interior of the evaporative emission control system is brought into the predetermined negatively pressurized state. Then, the second control valve is closed, followed by measuring a change in the tank internal pressure by means of a tank internal pressure sensor arranged at a suitable location in the fuel tank, and it is determined from the measured change in the tank internal pressure whether evaporative fuel has leaked to the outside through the evaporative emission control system (Leak Down Check) to thereby determine an abnormality in the evaporative fuel-processing system.
In the evaporative emission control system, while the third control valve is kept closed during the abnormality determination, it is kept open during normal purging to allow fresh air to be introduced into the canister through the air inlet port so that evaporative fuel temporarily stored in the canister is purged through the purging passage into the intake system of the engine.
To determine an abnormality in the evaporative fuel-processing system more accurately, it is desirable to measure an amount of change in the tank internal pressure with the lapse of time immediately after the interior of the fuel tank is made open to the atmosphere, and determine the presence of an abnormality by comparing between the measured amount of change and an amount of change in the tank internal pressure measured during the leak down check. That is, if a hole or the like is formed in the fuel tank, the interior of the evaporative emission control system cannot be negatively pressurized into the predetermined negatively pressurized state for carrying out accurate abnormality determination through the leak down check. Further, if evaporative fuel is generated in large quantities in the fuel tank, there occurs a large amount of change in the tank internal pressure during the leak down check. Consequently, an erroneous decision can be rendered that the system is abnormal, due to such a large amount of change in the tank internal pressure. Therefore, desirably, the abnormality determination should be made based upon both an amount of change in the tank internal pressure measured immediately after the interior of the fuel tank is made open to the atmosphere and one measured during the leak down check.
However, the above proposed abnormality determining method makes it a precondition to close the first control valve. Therefore, when the first control valve is defective, the abnormality determination cannot be made accurately. That is, when the first control valve is kept closed due to a fault, it is impossible to detect an amount of change in the tank internal pressure during the leak down check. As a result, the interior of the canister can be excessively negatively pressured.
Also when the first control valve is kept open due to a fault, it is impossible to detect an amount of change in the tank internal pressure occurring after the interior of the tank is made open to the atmosphere, so that accurate abnormality determination cannot be achieved.
Further, according to the earlier application, the leak down check is carried out after the second control valve is changed from an open state to a closed state. Therefore, if the second control valve is kept open due to a fault such as a short circuit in the electric system thereof, the interior of the evaporative emission control system can be excessively negatively pressurized.
Still further, according to the earlier application, when there occurs an abnormality in the electric system of the third control valve during the abnormality determination for the evaporative emission control system, the third control valve is kept closed even after normal purging is started, so that the interior of the evaporative emission control system becomes excessively negatively pressurized.